CANADA'S constitutional crisis is over. The threat of French-speaking Quebec breaking away to form a semi-sovereign nation has ended - for the time being.

After seven days of negotiations by the premiers of the 10 Canadian provinces and Brian Mulroney, prime minister of the Canadian confederation, Quebec got what it wanted - a promise by three holdout provinces to ratify the Meech Lake constitutional accord that had been agreed to unanimously by all 11 governments three years earlier.

``This is a happy day for Canada,'' declared Mr. Mulroney before signing an agreed-upon document Saturday evening.

One after another, the 10 premiers, including Quebec Premier Robert Bourassa, gave speeches, often in French and English, affirming Canadian unity. For the sake of national unity, the premiers of the three holdout provinces - New Brunswick, Manitoba, and Newfoundland - had to drop earlier demands for changes in the accord.

However, they did win promises from Quebec and the heads of the other governments to consider reform of Canada's appointed Senate and other constitutional changes in the future.

The goal of the western and Atlantic provinces is to win an elected Senate that would give them greater representation in Ottawa. Seating in the House of Commons is based largely on population. So Quebec and Ontario, where some 56 percent of Canadians live, often dominate Canadian politics.

The Meech Lake accord brings Quebec voluntarily within the Canadian Constitution. From confederation in 1867 until 1982, the Canadian Constitution was a statute of the British parliament. It was ``patriated'' to Ottawa in 1982. But Quebec, then ruled by a Parti Qu'eb'ecois government that sought greater separation from the remainder of Canada, did not sign the revised Constitution. This was primarily because the Quebec government feared that an addition to the Constitution, a Charter of Rights, might interfere with its right to promote French and limit the use of English in the province. Nonetheless, Quebec has been subject to the Constitution.